1. Field of the Invention
The present invention relates to two-terminal solid state four layer PNPN semiconductor diode switches deisiged to be fired by dV/dt.
2. Description of the Prior Art
A two-terminal, solid state, four layer PNPN semiconductor diode switch of the thyristor type may be fired by either the dV/dt or a high level of the applied anode to cathode voltage. This is in contrast to a three-terminal switching device that is designed to be fired by the application of a relatively low amplitude pulse, applied to a cathode or P-base. Two terminal switches are turned-off by commutation of the load current.
Conventional two-terminal diode switches when designed for high power, high speed requirements are prone to failure during turn-on because of localized heating of the device, and they also suffer switching losses. In order to avoid such localized heating, the semiconductor layers are designed to have uniform impurity densities throughut so that conduction is initiated simultaneously over its entire area. Such impurity density uniformity, however, for high power devices is difficult to achieve to the extent required. Such devices frequently turn-on slowly for a given dV/dt and fire in an uncontrolled manner.
Thus, it is desirable to have a two-terminal high speed, high power, solid state semiconductor switching device that has low switching losses, turns on reliably in a short period of time, and is not prone to failure because of localized heating, without the difficulty of achieving inordinate impurity density uniformity of the semiconductor layers.
In accordance with the present invention, a manner of achieving such results is to construct such a two-terminal four layer PNPN device having an auxiliary cathode emitter and a shunted main cathode emitter. Such auxiliary cathode emitter can be so constructed that the product of the dV/dt-induced capacitive current in the cathode base region within the confines of the outer edge of the auxiliary cathode, and the effective radial resistance of the cathode base beneath the auxiliary cathode emitter, and the rate of application of the dV/dt firing voltage is at least equal to the voltage on the cathode emitter junction that is capable of firing the device.
It is common practice to use auxiliary cathode emitters in three-terminal solid state four layer semiconductive thyristor devices, where a low amplitude gating pulse is applied to the central portion of the cathode base to fire the device. In such devices, the auxiliary cathode emitters are used to provide an improved dI/dt capability, (i.e., the rate of current increase or "turn-on" as a function of time) and to reduce the current carrying requirements of the gate electrode supply.
However, as far as is known, a two-terminal solid state four layer semiconductor diode switch prior to the present invention, has not been proposed that includes an auxiliary cathode emitter and a shunted main cathode emitter constructed as described herein to improve the switching losses, reliability, predictability, and turn-on response to dV/dt (i.e., the rate of applied anode voltage increase as a function of time).